Simple Calendar is free.
It will not suddenly become a paid app or stop working after a few days.
If you would like to hide ads, use all theme colors, and customize wallpapers and date background colors freely, the paid "Premium Service" (monthly or annual plan) is also available. The annual plan comes with a free trial. You can cancel at any time.
* If you previously purchased the one-time "Remove Ads" option, you can keep using the app without ads at no extra cost.